This invention relates to an optical displacement sensor for inspecting the sectional contour shape of a target object. In particular, this invention relates to such an optical sensor capable of inspecting with a high level of precision the sectional contour shape of a target object of a kind that is so formed that a contour image with uniform density of an optically sectioning surface is difficult to obtain because of its surface characteristics.
There has been known a prior art optical displacement sensor for such a purpose characterized as comprising a light projecting means for forming a slit beam of light from a light source and projecting this beam onto a target object at a specified angle, an imaging means for obtaining an image of a sectional contour line of the target object by an optically sectioning plane by using a two-dimensional image taking element from a different angle to photograph the position of incidence of the slit beam on the target object, and a measuring means for carrying out a specified process on the image of the contour line obtained by the imaging means and thereby generating a measured value and/or a judgment value. The direction of the sectional surface by the slit beam of light corresponds to the direction of the perpendicular scanning within the field of view of the two-dimensional image taking element. When the distance between the measuring device and the target object is changed, the image of the sectional contour line formed by the slit beam moves on the light receiving surface of the two-dimensional image taking element in the same direction as that of the horizontal scan line.
With such a sensor, data on a series of target points on a straight line on the surface of the target object can be obtained summarily without moving the projected light relative to the target object because a slit beam with a cross-sectional shape of a straight line is used, instead of spot light with a point cross-section. Thus, if such a sensor is used for the inspection of industrial products being transported along a production line, defective products can be quickly and dependably identified by accurately measuring various parts of their surfaces.
Industrial products come in different forms and shapes, including those having non-uniform surface conditions such as surface roughness and color such that the reflectivity is not uniform. For such a product, let Area A be a portion of its surface with high reflectivity and Area B be another portion with low reflectivity. If the two-dimensional image taking element is adjusted such that the image of Area A will come out clearly, the brightness (or clearness) of image of Area B will be insufficient. If the element is adjusted such that the image of Area B will come out clearly, on the other hand, the image of Area A will be too bright, and an accurate measurement may not be made.
FIGS. 41A and 41B illustrate problems of this kind. FIG. 41A shows on its left-hand side a cross-sectional side view of an object with a left-hand part with high reflectivity and a right-hand part with low reflectivity. Its image taken by a two-dimensional image taking element as explained above is shown on the right-hand side. In this example, an image of the left-hand part of the object is visible but there is not enough reflection of light from the right-hand part and it is not possible to measure the sectional contour line of the right-hand part. In the image on the right-hand side, the horizontal direction represents the direction of the sectioning surface of the slit beam of light, and the vertical direction corresponds to the direction of the height of the target object. The image of the incident beam on the target object should be a straight horizontal line extending all the way on the screen but its right-hand side is not visible in this example because of its low reflectivity. If measurements are taken on the basis of such an image, the height on the left-hand side can be measured but no measurement can be taken on the right-hand side.
FIG. 41B shows another target object having a groove with low reflectivity sandwiched between right-hand and left-hand parts with high reflectivity. The image taken of this object shows that the portion of the groove is missing, as indicated by a dotted circular line, because of its low reflectivity.
In the case of an object with sloped surface portions, the sloped portions tend to have smaller reflectivity, not allowing accurate measurements. FIG. 42A shows an example of such a target object. The image obtained from a two-dimensional image taking element will include three sections which are the image of the slit beam of light made incident on the target object. The image shows that the portions corresponding to the slopes are missing. Thus, the height of the left-hand and right-hand end portions and the higher center part can be measured from the image but a normal measurement cannot be made on the sloped portions because the brightness is not sufficient.
In the case of a target object with a curved surface, as shown in FIG. 42B, the curved portion tends not to reflect light sufficiently. Thus, the portion of the image obtained by a two-dimensional image taking element corresponding to the curved surface may be missing.